1. Field of the Invention
The present invention is related to an amplifier for electronic musical instruments and audio systems, and more particularly to an output signal converter for tube amplifiers which is installed on the rear side of a tube amplifier and capable of amplifying or attenuating output signals while maintaining advantageous output properties of tube amplifiers.
2. Prior Art
Because of progress in the field of semiconductor technology, vacuum tube amplifiers for electronic musical instruments and audio systems have been replaced by amplifiers based on semiconductor devices, such as transistors. However, tube amplifiers have particular output properties which cannot be reproduced exactly by amplifiers using only semiconductor devices and thus have persistent popularity so that a small number of tube amplifiers are still being produced, sold and used.
The disadvantages of vacuum tubes are well known. They have comparatively short lives, are broken more easily, are less reliable, bigger and more inconvenient to install than corresponding semiconductor devices. They are comparatively heavy, produce more heat and more expensive than similar semiconductor devices. In many cases amplifiers based on semiconductor devices must be used because the corresponding vacuum tube amplifier is not available.
Many types of amplifiers which mimic to some extent the output properties of tube amplifiers by using semiconductor devices have been devised and used in practice.
However, one output property that can be obtained by using tube amplifiers is the one obtainable when magnetic saturation in an output transformer installed on the backside of a tube amplifier and counter electromotive force generated in a speaker affect the performance of a tube amplifier. Therefore, in the prior art there is a limit to the ability of semiconductor devices to reproduce the output properties of tube amplifiers. In fact, no amplifiers which can accurately imitate the output properties of tube amplifiers are known in the prior art.